The use of structural seals for sealing fluids under pressure, for example in gas turbine engines, in order to prevent fluid leakage is well known in the art. An assortment of structural seals have been developed with varying shapes such as the “C” shape, “U” shape, and “E” shape, as illustrated in FIGS. 1a-1c, respectively. In conventional applications the seals illustrated in FIGS. 1a-1c are installed in a cavity in a compressed condition by compressing the free height (ho) of the seal cross section to an insertion or compressed height (hc) see, for example, FIG. 2. In such conventional applications sealing is achieved via elastic spring-back under a combined load of both compression and fluid pressure. Such seals are referred to as compliant because they are able to be compressed and then expand in order to achieve sealing. The amount of seal compression, defined as the difference between the seal's free height and compressed height, i.e., ho−hc, provides a sealing load during use which is further augmented by the fluid pressure. As will be appreciated, the cross sections of these structural seals are designed to be compliant i.e., the seal cross sections are able to flex as the sealing surface, for example a flange, moves with respect to the second sealing surface (for example a second flange) while maintaining the seating load against the flanges, and hence, the cavity pressure (FIG. 2).
The flange movement is quite common in any high temperature pressurized chambers such gas turbine engines, where various segments are held together with the conventional structural seals placed in between. As the engine components heat up and cool down at differential rates, the flanges of different segments move both radially and axially with respect to each other. These movements are known to cause wear of the surface of the seal 1 and the contacting flanges 2 as the two surfaces under high mechanical contact pressure (Pmc) try to slide with respect to each other (FIG. 3). To minimize wear of the structural seals and the flanges, it is known to coat the seals with wear resistant coatings on the outer surface of the seal. Conventionally, the wear resistant coatings are applied by thermal spray techniques such as plasma spray or High Velocity Oxy Fuel (HVOF) of cermet (ceramic-metal composite) powders. Thermal sprayed coatings are generally very hard (>1000 VHN) but also less ductile (hence more brittle) than other coatings. This brittle nature can lead to coating failure by delamination during flexing of the seal in use. The delamination can then lead to three body wear of the thin compliant seal structure as particles (third body) can become trapped between the surfaces, which can result in seal failure.
When utilizing conventional application techniques such as thermal spraying, the seals need to be masked prior to spray in order to decrease the risk of coating inside the convolutions of the seal (“ci”), which is undesirable. The outer convolutions (“co”) are typically coated with a layer in the range of about 0.010″-0.020″ in thickness which generally has high surface roughness in the as-sprayed condition. The seals may be mechanically polished to yield a thinner and smoother coating, which is desirable for seal applications, for example of about 0.005″. In conventional spraying processes the compliant seal structure is supported within a specialized fixture in order to minimize any changes in the seal free height that could adversely affect seal performance. Because of the high hardness of these thermal sprayed coatings, the flange counterfaces that contact the seal should also be coated with thermal spayed coatings with similar hardness and then mechanically ground in order to prevent undue wear of the flange from contacting the thermally sprayed seal coating. In addition, the high hypersonic speeds at which the thermally sprayed coatings are applied can cause the seal to collapse, resulting in rejection of the seal.
While thermal sprayed coatings help prolong seal life by minimizing wear, they do suffer from a variety of shortcomings including the additional labor and cost associated with the mechanical polishing after thermal spray in order to achieve the desired thickness and surface finish; the high hardness of the coatings which can require additional thermal sprayed coating and grinding of the flanges; and the special equipment and labor associated with masking and spraying the seals to selectively coat and minimize change in the seal free height and the cost of rejecting or reworking seals that undergo unacceptable changes in seal height. In addition, the brittle nature of thermal sprayed coatings containing extremely high volume fraction of ceramic phases can lead to coating failure by delamination resulting from seal flexing and even to seal failure as detailed above.
In view of the foregoing, there is continued effort to develop suitable coatings for compliant structural seals that can successfully seal against uncoated flanges while being applied in a cost effective manner.